The invention relates to a volute shaped pump casing according to the preamble of claim 1 and to a centrifugal pump including such a volute shaped pump casing.
Volute casing pumps are very common. Their characteristic feature is the volute-shaped pump casing which, as a rule, makes this pump type recognizable from the outside. Volute type casings can be built as part of a single stage or multistage pump arrangement. In some multistage pumps a volute shaped casing is provided only for the last stage. Single suction and double suction, double volute casing pumps are both used frequently.
A volute shaped casing generally includes a chamber designed to house at least one impeller being usually of the radial or mixed flow type and mounted on a shaft for rotation when driven by a motor. The casing further includes a volute shaped chamber to collect pumped medium and a channel and discharge section to guide the medium out. The discharge can be arranged tangentially to the volute casing, or arranged radially by providing a swan neck. A suction channel section is favorably arranged axially in case of bearings arranged only at one side of the impeller, and radially or tangentially in case of bearings at either side of the impeller.
In its simplest embodiment of a single volute, the casing can be broadly sub-divided into two main sections consisting of a downstream chamber section including a volute shaped chamber and the upstream channel and discharge section. The plane or section at which the volute and channel meet is generally defined as the throat. The leading edge of the throat which separates or guides the flow from the chamber into the channel is designated cutwater lip or cut water and for any given length the top and bottom surface extending beyond the lip is termed the tongue. In the case of a casing with a plurality of volutes and flow channels disposed around an impeller the number of lips will usually be equal to the number of volutes or flow channels. I.e., in case of a double volute there will be two lips.
The conventional pump casing with double volute and double discharge is arranged in such a way that an outer or long channel connected to a first volute is wrapped around a second or inner volute and over a short channel connected to the second volute so that the discharge of the pumped medium is made through a common discharge nozzle which can e.g. be provided in a flange. This provides effective pumping means but imposes a large area envelope in radial direction and, thus, an increased height which is disadvantageous for pump arrangements requiring a more compact solution.
A current example of such a pump arrangement is a two-stage pump with two impellers mounted back-to-back on a shaft and separated by a centre-bush secured to the shaft, with one impeller mounted in a stage casing and the other in a discharge casing. The intake to the discharge casing is made via a top and a bottom crossover channel which respectively cross over and under the stage casing and the discharge casing. The cross sections of the crossover channels join at the intake opening of the discharge casing.
The height imposed by the conventional double volute casing signifies a disadvantageous overall pump height and a large distance between the crossover channels and the centre-bush, with the casing part of the centre-bush being made up of a large mass of material. This furthermore results in additional large pockets of material mass between the crossover channels and the neighboring contours of the casing. Thus, from a cost standpoint, the greater height of the conventional double volute casing leads to increased mass in the described multistage embodiment and therefore to a more expensive solution.